[Dialogs] Replace placeholder graphics with proper, approved dialog graphics

Currently dialogs only have default placeholder graphics which do not have the necessary approvals for release. We need to source new images and replace the existing ones.

Here is the list of images we currently use that would need replacement:
 * A 'More Details' image. Currently we use an arrow pointing downwards (to suggest that the dialog can be expanded)
 * A 'Less Details' image. Currently we use an arrow pointing upwards (to suggest that the dialog can be collapsed)
 * A 'warning' image
 * A 'confirm' image
 * An 'error' image
 * An 'information' image

The four main images (the last four) are are 48x48 pixels in size, but this is not a hard requirement. I have attached the current images I have used as placeholders into this jira. They are from the Oxygen icon set, which when I retrieved them was licensed under the Creative Common Attribution-ShareAlike 3.0 License (http://creativecommons.org/licenses/by-sa/3.0/). For reference, these dialog icons were taken from http://websvn.kde.org/trunk/KDE/kdebase/runtime/pics/oxygen/?pathrev=706996